Think of a historian like a police detective. They come in after the event and piece together the clues to work out what happened. Like a detective they will gather eye-witness accounts from people at the scene or who have some memory after the event of what happened. They will look for written evidence, like a newspaper article or a letter, which tells the story and give some clues. They will consult their specialist scientists for clues, in heritage those scientists are conservators.
Sometimes the clues leave a clear trail and the story can be pieced together with a good deal of certainty. Other times, the clues, the oral stories, the memories and the written evidence are not so clear or may never have been there in the first place. Memories dim and flaws can creep in to any story, regardless of whether your grandmother tells it or you read a journalist’s account in the newspaper.